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A way to discover the true species diversity of single-celled organisms?

What is the true diversity of organisms on our planet? That is the question that keeps many scientists awake at night. Interestingly, until now many more macroscopic organisms have been described than the very smallest ones, invisible to the naked eye. But one would probably assume that those tiny organisms will be more abundant as they have obviously had much more time for their differentiation.


Morphological diversity of Asterochloris chloroplastids. Photo by Pavel Škaloud. 

The problem lies mainly in the fact that it is much more difficult to distinguish between single-celled organisms that have relatively few morphological characters than those big, multicellular organisms.

It is not an exception that single-celled and microscopic organisms have defined, in their description, a very wide variation of conditions and habitats they inhabit. And is this not due to the fact that there are many species that are much more limited environmentally and that we have been unable to distinguish morphologically? If microscopes are not a sufficient tool we have to use other methods, such as molecular ones.

The Journal of Phycology published a study written by Pavel Škaloud and his colleagues, in which the group of phycologists and lichenologists dealt with the diversity of one of the most common lichen photobionts – alga from the genus Asterochloris. In total they processed the data of 250 isolates originating from their own collections in Central and Eastern Europe and from borrowed cultures. Asterochloris is not very picky in terms of the fungal partner – it enters into a symbiotic association with more than twenty genera of fungi, with which it creates various types of lichens. Therefore, it was important to record the lichen species in which the algae occurred as well as the lichen environmental conditions.

Regarding molecular methods, authors used many molecular markers to determine the genetic variability. These were the four loci: slowly evolving SSU rDNA and rbcL genes and rapidly developing ITS rDNA and actin introns.

In addition to molecular methods the authors focused on the observation of the shape of chloroplasts in cells using confocal microscopy. With this tool you can view the structure of these organelles in 3D and compare individual algal lineages among themselves. The shapes of chloroplasts can be really diverse, even within a single species (Fig.).

Combining all the available data resulted in the description of 6 new species of this algal genus. Species actually differ from each other in the shape of chloroplasts, in preferences to environmental conditions and fungal partners and of course on the genetic level. It is noteworthy that these species can only be genetically separated from each other by analysing the rapidly evolving parts of the genome – ITS and actin. The slower evolving sequences, such as SSU, could not be used to distinguish these species. This finding is very important, because this sequence is currently very widely used to estimate the total diversity of protists on the basis of environmental sequencing. It is more than likely that because of this much of the existing algal diversity escapes our attention.

The icing on the cake of the study is the observation of sexual reproduction in Asterochloris. Because it is almost a universal opinion that algae of the class Trebouxiophyceae, which includes Asterochloris, do not reproduce sexually at all and that they lost this capability in evolution. They occur mostly in an aero-terrestrial environment where they are stressed by a lack of water and are rarely active. Perhaps because their sexual reproduction has only been very rarely observed it was considered that the majority of the representatives are not capable of it. However, relatively recent studies have shown that genes for meiosis occur in the genome of these algae, indicating the ability to reproduce sexually. This work also showed that the persistent image of asexuality of terrestrial green algae is wrong. It seems that we are slowly crossing a historic milestone and that a world of hitherto undescribed organisms that are still waiting to be discovered will unfold before us. We definitely have something to look forward to.


Published: Oct 06, 2015 01:20 PM

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